1. Field of the Invention
The present invention relates to a transfer system for transferring a load such as a heavy object to be processed, and a transfer pallet on which the load is placed.
2. Description of the Related Art
FIGS. 27 to 31 show transfer systems known in the art.
Referring first to FIGS. 27 to 29, a conventional transfer system includes a truck 1. A table 2 is connected to a truck body 3 by a vertical support 2'. Wheels 6 are rotatable about a horizontal shaft which is mounted to the truck body 3. Reference numeral 4 designates a guide having a U-shaped cross section. A pair of rails 5 are fixed to the inner walls of the guide 4. The wheels 6 of the truck 1 are supported on the rails 5. Wheels 7 are each rotatable about a vertical shaft which is connected to the truck body 3. The wheels 7 are in contact with the inner side walls of the guide 4. Reference numeral 8 designates a secondary conductor of a linear motor. Primary coils 9 of the linear motor are fixedly mounted on the bottom of the guide 4 and are spaced apart from one another in the direction in which the truck is advanced. Reference numerals 10 designate stations. A turning section 11 of the guide 4 changes the direction of advancement of the truck. With this arrangement, the wheels 6 are placed on the rails 5, and the wheels 7 are brought into contact with the side walls of the guide 4. A load (a heavy object to be processed) is placed on the table 2. The truck 1 is moved from one station 10 to another station 10 along the guide 4 when the linear motor primary coils 9 are energized.
FIGS. 30 and 31 show another transfer system known in the art. Reference numeral 14 designates 14 a base. In air duct 15 is seated on the base 14. Reference numeral 12 designates a top plate of the air duct 15. A number of nozzle holes 13 are formed in the entire surface of the top plate 12. Linear motor primary coils 16 are mounted within the air duct 15. Reference numeral 17 designates a pallet including a linear motor secondary conductor on its lower surface. Reference numeral 18 designates a pressurized air supply source. The base 14 and the air duct together form an air table. Air under pressure is supplied from the pressurized air supply source to the air duct 15 and is then injected from the nozzles 13 to float the pallet 17 together with a load above the top plate 12 of the air duct 15. The pallet 17 together with the load is transferred from one station to another station when the linear motor primary coils 16 are energized.
Conventionally, a transfer system employs a pallet to carry a light load of several kilograms to several hundred kilograms. The pallet together with the light load is floated on an air table and moved by a linear motor as shown, for example, in FIGS. 30 and 31.
In the prior art transfer system shown in FIGS. 27 to 29, the table 2 is attached to the truck 1 by a single post or support 2'. This causes the table 2 to swing and move out of alignment with the guide 4. Thus, a load on the table can not be moved in parallel to the guide 4.
Also, if the section 11 of the guide 4 has a small radius of curvature, a pair of front wheels 6, the four wheels 7 and a pair of rear wheels are oriented as shown in FIG. 28. This inhibits a change in the direction of movement of the truck 1. To this end, the guide 4 is constructed in a factory so that the section 11 has a large radius of curvature. This detracts from the flexibility of the transfer system. In addition, the truck 1 can not be stopped in a predetermined position. This detracts from the operability and safety in a production line.
In the prior art transfer system shown in FIGS. 30 and 31, a large pressurized air supply source is required to inject air under pressure from a number of nozzle holes 13 of the air duct (or air table) not only before each pallet 17 passes, but also after the pallet has passed. This results in an increase not only in the cost of the system, but also in the consumption of the pressurized air and thus in the operating cost of the system.
In the prior art transfer system incorporating the air table and the linear motor, a pallet must be rigid and heavy to transfer a heavy load, for example, of several tons (about two tons) at a high speed (2 meters/sec). To transfer such a heavy pallet at a high speed it is necessary to employ a large linear motor and to effect a high flow rate of pressurized air. However, this leads to the necessity of using a large transfer system.
In order to improve production efficiency, a pallet is transferred to a press machine in a production or press line while being floated by air under pressure. A force is then applied to press a load on the pallet. The pallet must be rigid to accommodate this force. This leads to the necessity of employing a heavy pallet. To transfer such a heavy pallet, it is necessary to provide a large linear motor and to effect a high flow rate of air under pressure. This results in the necessity of employing a large press system.